Human systemic sclerosis (SSc) is an autoimmune-like disease that results in excessive production of collagen in the skin as well as tissues such as kidney, heart, and lung. If this fibrosis is extensive it can prevent normal organ function. Fibrosis is a feature that is shared with other syndromes such as severe asthma and COPD, and therefore preventing the onset of fibrosis or blocking continued fibrosis is of great significance to a number of damaging diseases. Collagen deposition is thought mediated by deregulation of epithelial cells, macrophages, and fibroblasts, and inflammatory cells such as Th2 cells and eosinophils may play roles in promoting the activity of these cell types. Although cytokines such as TGF-, IL-13, IL-4, and TNF are acknowledged to contribute to end-stage pathology, new and novel targets for therapy are warranted that may broadly suppress the activity of many of the inflammatory cell types that contribute to fibrotic disease. We recently reported in several models of severe asthma that LIGHT, a TNF superfamily protein, that can be expressed on a number of cell types including activated T cells, dendritic cells, and inflamed epithelial cells, was responsible for mediating collagen production, smooth muscle hyperplasia, and overall fibrotic symptoms. This activity was mediated through binding to two TNFR superfamily receptors, the herpes virus entry mediator (HVEM) and the lymphotoxin beta receptor (LTR). We will test the novel hypothesis that LIGHT also controls fibrosis in multiple inflammatory situations and will be responsible for collagen deposition, skin thickening, and interstitial fibrosis of internal organs. We have evidence that HVEM and LTR are expressed or induced on all of the cell types thought to mediate fibrotic activity, namely epithelial cells, macrophages, and fibroblasts. We also have found that patients with pulmonary fibrosis due to systemic sclerosis have T cells in the lungs that express LIGHT, which correlates with clinical reports of enhanced levels of soluble LIGHT in bronchoalveolar lavages from similar patients. This proposal will focus on three mouse models of systemic sclerosis that result in fibrosis in various organs and recapitulate many of the features exhibited by human patients. Using the tight- skin mouse that spontaneously develops skin fibrosis; a minor histocompatibility model of GVHD that also results in skin fibrosis as well as fibrosis of internal organs; and bleomycin injection that induces pulmonary fibrosis, we will test whether mice or cells lacking LIGHT, HVEM, or LTR cannot succumb to sclerosis-like fibrotic disease. We will additionally test if therapeutic blockade of LIGHT interactions ameliorates inflammation and pathology associated with scleroderma and systemic sclerosis.